The present invention relates to a method and a device for safely monitoring the rotary motion of a shaft. The device comprises measuring means for recording a first and a second signal, wherein the first signal represents the motion characteristic of an angular position of a first reference point of said shaft and the second signal represents the motion characteristic of an angular position of a second reference point of said shaft, wherein said first and said second reference points are spaced apart one from the other by a rotary angle of said shaft, and wherein said device further comprises an evaluator which receives said first and said second signals.
An example of a rotary motion sensor that supplies such before-mentioned signals is, for instance, a resolver. Resolvers are well known in the art for recording rotary motions. They comprise a rotary transformer, the rotor of which is connected to the shaft which is to be monitored, and the stator of which comprises two separate windings arranged on the outer circumference of the shaft with an angular displacement of 90xc2x0 one relative to the other. Being coupled in transformer fashion, the two stator windings are fed with a signal that is supplied via the rotor winding. The signals present at the output of the stator windings then represent the before-mentioned first and second signals. The function of a resolver will be described hereafter in more detail with reference to FIGS. 2 and 3.
For evaluation of resolver signals, complete circuits are known and available as integrated circuits (IC), as for example the integrated circuit No. AD2S80A available from Analog Devices. That IC is a so-called resolver/digital converter which calculates the rotary angle of the shaft from the resolver signals and presents the result as a digital numeric word at its outputs. In addition, a signal proportional to the rotary speed of the shaft is presented at an other output. Thereby, the known module generally allows to monitor the rotary speed of a shaft. However, this monitoring is not safe, since, for instance, a short circuit between two windings occurring in one of the stator windings cannot be detected reliably. Likewise, internal functional errors occurring in the integrated circuits cannot be safely detected, either. It is therefore possible, for example, that the integrated circuit fails due to some errors, thereby supplying a signal that indicates the shaft to be monitored at a standstill, although the shaft actually rotates at high speed.
Failsafe monitoring of rotary motions of shafts is necessary, for example, when the rotating shaft presents a risk for machine personnel or other machine elements. This is especially the case in situations where an operator has to work within the danger zone of a rotating shaft during set-up operations. The evaluation circuits for resolver signals known heretofore are not suited for safely monitoring the rotary motion of a shaft.
In order to ensure safe monitoring in such a case, it is known to arrange at least one additional rotation sensor at the shaft, in addition to the resolver. Usually, even two separate rotation sensors, for example incremental pickups, are employed today. In general, these additional rotation sensors merely serve to ensure safe monitoring of the rotary motion of the shaft. In addition, a resolver is frequently used as a third sensor for adjusting the rotary motion of the shaft during normal operation. An example of such a known arrangement will be explained hereafter with reference to FIG. 1. As an evaluation device for safely monitoring of zero-speed conditions, or for safely monitoring a controlled low rotary speed of the shaft, a standstill monitoring relay called PDZ available from Pilz GmbH and Co., Ostfildern, Germany, may be used, for example; this monitoring relay is described in the company""s Operating Instructions No. 19 161.
Another example of a device for safely monitoring a speed is described in DE-A-38 19 994. Said known device uses two separate inductive proximity pickups as sensors.
Using a plurality of additional sensors for safely monitoring the rotary motion of a shaft is expensive and negatively affects the costs of an installation to be monitored.
It is an object of the invention to provide a device and a method of the before-mentioned kind which allow the rotary motion of the shaft to be safely monitored, even with only a single rotation sensor, especially a singly resolver.
It is another object of the invention to provide an arrangement having a rotating shaft, the rotation of which is controlled by a drive control, and a safety device for monitoring the rotation of the shaft, wherein said safety device is capable of safely stopping the rotational movement of the shaft, when an error is detected.
According to one embodiment, these and other objects are achieved by a device of the before-mentioned kind, wherein the evaluator comprises a comparator which is capable of comparing instantaneous values of the first and the second signals using a predefined geometric relation.
According to another embodiment, these objects are further achieved by a method of the before-mentioned kind, wherein instantaneous values of the first and the second signals are compared using a predetermined geometric relation.
According to the invention, instantaneous values of the first and the second signals are compared one with the other. Due to the fact that the signals are recorded at different reference points of the same shaft, there is a predetermined relation between such signals. As long as the operation of the monitoring device is free from errors, a comparison taking said relationship into account must always lead to a predictable result. Whenever a comparison between the two signals yields a different result than would be expected, an error has occurred in the monitoring device.
In this connection, the term xe2x80x9cgeometric relationxe2x80x9d is understood as generally describing the predetermined relation between the instantaneous values of the two signals, which can be determined based on the geometric position of the reference points one relative to the other. This relationship may be graphically illustrated as a locus curve in a plane whose coordinates are defined by the instantaneous values of the two signals. If the actual shape of the locus curve derived from the recorded instantaneous values deviates from the expected shape of the locus curve, an error has occurred in the monitoring device.
It should be noted that, in principle, the method according to the invention can be employed for safe monitoring also when a plurality of separate sensors are used for recording the first and second signals. However, this is not required, since the method can be used with a single resolver for recording the rotary motion, because the described first and second signals are supplied by the resolver as such. Any possible error, as for example a short circuit between windings in one of the stator windings of the resolver, or failure of a component of the evaluator that will be described in more details in the following, any such possible error leads to a different result in the comparison between the instantaneous values as that one would have expected due to the predetermined relation between the two signals. Consequently, the method according to the invention can be employed for safely monitoring the rotary motion of a shaft with a single rotation sensor only, namely a resolver. The additional use of further sensors, such as incremental pickups, as usual heretofore, is no longer necessary.
According to a preferred embodiment of the invention, the comparator is capable of comparing concurrent instantaneous values of the first and the second signals.
This feature advantageously simplifies the predetermined relation between the instantaneous values of the first and the second signals so that the comparison between the instantaneous values can be realized more easily. This consequently reduces the effort required for both the development and the production of the inventive device. The corresponding method can be carried out more quickly in this way.
According to a further embodiment of the invention, the first and the second reference points are spaced apart one from the other by a rotary angle of 90xc2x0.
This feature likewise simplifies the predetermined relation between the instantaneous values of the two before-mentioned signals. Consequently, the effort and the costs of the device are likewise reduced by this feature. The corresponding method can be carried out more easily and more quickly.
According to another embodiment of the invention, the signal paths for recording the first and the second signals are identical under signal-processing aspects.
Identical under signal-processing aspects means that identical output signals will be achieved, whenever identical input signals are fed into the respective signal paths. Of course, however, the signal paths may be implemented using differing components, for example from different manufacturers. The only requirement that has to be fulfilled is that the first and the second signals must be influenced in identical fashion. The feature also contributes toward making the comparison between the two signals easier and quicker. In combination with the feature described before, this embodiment of the invention results in the geometric relation being a circular function which can be evaluated in a very easy manner both analytically and arithmetically, as will be shown hereafter.
According to a further embodiment of the invention, the measuring means comprise a resolver whose stator signals are the first and the second signals.
This feature takes up the advantage already mentioned before, namely a single sensor is required only for safely monitoring the rotary motion of the shaft, when a resolver is employed. Accordingly, an especially simple and low-cost structure is achieved by this embodiment of the invention. However, it should be stressed once more that, in principle, the method may also be carried out with the aid of separate sensors for recording the first and the second signals. In addition, other sine/cosine signal pickups may be used instead of a resolver for carrying out the method according to the invention. Compared with such pickups, however, a resolver provides the advantage of being extremely robust so that it offers high error resistance from the very beginning. In addition, resolvers are already in use in many systems as sensors for the drive control or drive governing systems. Thus, the use of the resolver signals, which are available anyway, allows further cost savings in the implementation of the inventive device.
In a further embodiment of the invention, the comparator comprises a digital calculating unit in which the geometric relation is stored as a calculating rule using the instantaneous values as inputs.
A digital calculating unit of that kind is particularly a digital signal processor. Such a processor is especially optimized for carrying out computing operations. Since the predetermined relation between the instantaneous values of the first and the second signals can be described, generally, by a mathematical formula, the comparison between the instantaneous values can be carried out with particular accuracy with the aid of such a processor or, to say it more generally, with the aid of a digital calculating unit. If the geometric relation is a circular function, the sum of the squares of the instantaneous values of the first and the second signals must always yield a constant value as long as no error occurres in the monitoring device. Using a digital calculating unit, especially in the form of a digital signal processor, it is possible to detect even very small deviations of the instantaneous values of the first and the second signals from their desired values.
According to an alternative embodiment of the feature described above, the comparator comprises a memory in which admissible combinations of instantaneous values corresponding to the geometric relation are stored in such a way that any incorrect combinations of recorded instantaneous values can be detected.
This feature may be implemented, for example, by storing the correct and therefore admissible combinations of instantaneous values of the two signals in an allocation table. If a combination of recorded instantaneous values is not contained in that table, it is automatically detected as an incorrect combination, and a corresponding error signal can be generated. This feature provides the advantage that it can be implemented in a very simple and especially low-cost manner. in particular, the costs of a relatively expensive digital signal processor can be avoided in this way. An additional advantage of that feature is the fact that the evaluation process can be carried out very quickly, when the number of admissible combinations of instantaneous values stored is not too high.
According to a preferred embodiment of the feature described before, the stored combinations of instantaneous values define two-dimensional admissible interval fields each of which adjoins neighboring admissible interval fields.
In more concrete terms, this means that the stored combinations of admissible instantaneous values define interval areas, and the locus curve of the recorded instantaneous values stays within said interval areas as long as the device operates without any error. If any of the locus points defined by two recorded instantaneous values comes to lie outside the admissible interval fields, this is an indication of an error occurred in the monitoring device. The term xe2x80x9cadjoiningxe2x80x9d is to be understood here to also include overlapping of neighboring interval fields in the bordering area. The feature mentioned above advantageously permits the shape of the plotted locus curve to be continuously monitored.
An especially advantageous solution is achieved if a maximum of two interval fields adjoin one another at each point of the specified locus curve. In this case, the two-dimensional interval fields are defined relative to the expected shape of the locus curve, such that each locus point of the locus curve may come to lie in the boundary area of two interval fields at maximum. A boundary point, where three or even more interval fields adjoin each other, is avoided in this case. This feature provides the advantage that any combination of recorded instantaneous values may cause an ambiguity with respect to two admissible interval field at maximum, but there will never be an ambiguity with respect to three or more admissible interval fields. The effort and, consequently, the time required for carrying out the comparison are reduced therewith.
According to a further embodiment of the invention, the device comprises means for determining at least the rotary speed of the shaft.
In addition to that, the before-mentioned means may also be capable of determining the respective angular position of the shaft and/or its sense of rotation. Such determination may be effected with the aid of methods known per se, based on the evaluation of the first and the second signals. This feature advantageously enables an additional error check based on the values so determined, namely by comparison of these values with corresponding values provided by a separate speed control unit. Alternatively, this feature may contribute to avoid the requirement of an additional speed control unit.
According to still another embodiment of the invention, each of the measuring means for the first and the second signals has a separate and independent recording channel.
This feature is advantageous, since the signal paths for the two before-mentioned signals are completely separated one from the other with the result that the failsafety of the device is even further improved. It is particularly preferred in this connection if the two separate recording channels have a diversity-based structure, i.e. they use different components. In this case, the probability that the recorded instantaneous values of the first and the second signals might be erroneously varied in the same way and at the same time is even further reduced.
According to a further embodiment of the invention, the device comprises two comparators, one redundant to the other, whose comparison results are supplied to a monitoring unit.
This feature further improves the failsafety of the device, because it allows a failure occurring within one of the comparators reliably to be detected. According to an especially preferred solution, the two mutually redundant comparators have a diversity-based structure. More preferably, the monitoring unit likewise has a two-channel structure, wherein each of the two channels is assigned to one of the two mutually redundant comparators.
According to another embodiment of the invention, the device is designed as a modular add-on unit for an additional rotary-motion evaluation device.
This feature provides the advantage that the inventive device can be retrofitted in a simple and low-cost manner even in existing installations. In addition, the inventive device can be used in establishing new installations in combination with identical and/or other rotation pickup devices, especially with standard components. This permits the inventive device to be employed in a generally low-cost manner.
It is understood that the features recited above and those yet to be explained below can be used not only in the respective combination indicated, but also in other combinations or in isolation, without leaving the context of the present invention.